The invention relates to the fabrication of stainless steel strip. To be more precise, it relates to an installation for producing stainless steel strip combining all fabrication process steps from casting to cold rolling.
Cold-rolled stainless steel strip to be shipped to customers in coiled form is usually made by the following process:
continuously casting slabs approximately 15 to 25 cm thick from liquid steel in a bottomless mould with cooled fixed walls,
hot rolling the slabs to form a strip a few millimeters thick,
optional initial annealing of the strip,
descaling and cleaning the surface of the strip by mechanical, chemical or electrochemical means or a succession of such means,
cold rolling the strip to virtually its final thickness, generally in a rolling mill including a plurality of mill stands,
annealing,
pickling, and
passing through a skin-pass rolling mill before final packaging in coiled form.
The cold rolling can also be carried out in several steps separated by intermediate treatments.
Annealing and pickling after cold rolling can be replaced by bright annealing, depending on the required surface state and appearance of the strip.
The above operations are carried out on installations which are mostly separate from one another, and this necessitates multiple packaging operations, such as intermediate coiling operations, and transferring coils from one installation to another.
However, it is known in the art to perform the steps of transforming the hot-rolled strip to cold-rolled coiled strip ready for sale on a single installation, in which the steps are performed in-line. The document EP 0 695 808 describes one example of this.
The above document also mentions the possibility of carrying out the above transformation steps not on strip produced from hot-rolled slabs but on strip produced by an installation for direct continuous casting of thin strip a few millimeters thick from the liquid metal, for example by casting the liquid metal between two internally cooled horizontal axis rolls. Casting can optionally be followed by light hot rolling performed on the same line as the casting or on a completely separate installation, with the particular aim of limiting the surface roughness of the strip and encouraging recrystallization of the metal to prevent xe2x80x9cropingxe2x80x9d during subsequent transformation of the strip. An annealing furnace can also be placed downstream of the hot rolling mill and upstream of the installation for coiling the strip.
In any event, the production line for cold-rolled stainless steel strip is long and represents a heavy investment. It is also costly in terms of energy consumption because of the multiple operations of transporting intermediate product (slabs and coils) between installations which are not always on the same industrial site and because of the reheating of the product which is necessary before several of the cited operations to raise it from ambient temperature to the required treatment temperature. What is more, storing intermediate product pending processing takes up space and immobilizes metal.
Using an installation for direct casting of thin strip reduces these drawbacks and eliminates the hot rolling step or at least considerably reduces its scale. What is more, if hot rolling is performed in-line, without intermediate coiling, the latent heat of the strip that has just been cast is a benefit.
Problems with the surface quality of the strip remain, however. It is difficult always to avoid to a sufficient degree the presence of encrusted scale on the surface of the strip obtained after treatment. Also, on the more fragile grades of steel, such as SUS 409 ferritic stainless steel, surface cracks frequently appear on the finished strip and render it unusable.
The object of the invention is to propose a method of fabricating cold-rolled stainless steel strip which regularly achieves a very good surface quality of said strip and a fabrication installation suited to the use of the method.
With the above objectives in view, the invention provides a method of fabricating thin stainless steel strip in which the following steps are carried out successively and continuously on a single processing line:
continuously casting a stainless steel strip directly from liquid metal,
descaling said strip,
cold rolling said strip,
annealing and pickling or bright annealing said strip,
finishing said strip, for example by passing it through a skin-pass rolling mill, and
coiling said strip.
Said strip can optionally be hot rolled immediately after casting it and this hot rolling can be followed by annealing of said strip.
The invention also proposes an installation for producing cold-rolled stainless steel strip, the installation including, in succession:
an installation for continuously casting thin strip directly from liquid metal,
a descaling installation,
a cold rolling installation,
an annealing-pickling installation or a bright annealing installation,
an installation for finishing the strip, such as a skin-pass rolling mill, and
an installation for coiling the strip, all said installations being disposed on a common continuous line and at least some of them being separated by a strip accumulator.
The installation optionally also includes a hot rolling mill downstream of the continuous casting installation. The hot rolling mill can be followed by an annealing installation.
The casting installation can be of the xe2x80x9ctwin-roll castingxe2x80x9d type.
The invention clearly consists of continuously executing on a single production line all the process steps for fabricating cold-rolled stainless steel strip from thin strip cast directly from liquid metal, from the casting step through finishing steps to packaging the strip in the form of a coil ready for shipping to the customer.
The inventors have realized that the residual encrustation of scale and the surface cracks found all too often on cold-rolled strip made on conventional discontinuous installations are considerably reduced by eliminating all the intermediate coiling operations that the strip normally has to undergo before it is transferred from one shop to another. Coiling inevitably subjects the strip to tension, which can be sufficient to damage particularly fragile grades, either during coiling itself or during the period in which the cooled, and therefore relatively unmalleable, strip remains coiled. Also, the time the strip spends in the form of tightly wound turns contributes to deeply encrusting into its surface layers any residual scale remaining on its surface if it is imperfectly cleaned before coiling. Eliminating all the intermediate coiling operations, which is allowed by the method according to the invention and the entirely continuous production line according to the invention therefore eliminates major causes of deterioration of the surface quality of the strip.
Other advantages of the invention may also be cited.
Each coiling operation considerably deforms the beginning and the end of the strip constituting the coil. It therefore renders those parts unusable and, what is more, they could damage the parts of the installation that they pass through in the next process step. It is therefore essential to remove them. Eliminating intermediate coiling operations in the method according to the invention therefore reduces the quantity of metal rendered unusable during the fabrication process and likewise the number of installations for shearing the beginning and end of the strip.
The processing line according to the invention can be relatively compact and in any event has a smaller footprint than the discontinuous (and often geographically dispersed) installations that it replaces. What is more, it becomes a simple matter to group together on one site and close to the line various shops whose functions are common to different parts of the installation.
The operations of transferring coils from one installation to another are eliminated, saving considerable time in the total duration of the fabrication process. Also, the usual coil transfers have the drawback of immobilizing metal, which is not yet ready to be shipped to customers, and therefore immobilizing capital on which no return is yet possible. What is more, the coils awaiting processing must be stored and the storage areas increase the size of the factory. The invention enables the factory producing the cold-rolled strip to operate on a xe2x80x9cjust in timexe2x80x9d basis with the usual economic advantages of that type of operation.
The use of a conventional installation for continuously casting slabs produces large quantities of highly adherent scale after casting and before and immediately after hot rolling. Experience shows that direct continuous casting of thin strip reduces the total quantity of scale formed after casting, and after optional hot rolling, especially if an inert atmosphere is used in the lower part of the casting plant, for example using a cover under which a non-oxidizing or even reducing atmosphere is maintained. What is more, the scale that does form is less strongly adherent to the surface of the product than the scale formed in conventional casting and hot rolling installations, which is probably due to the shorter time for which the product remains at high temperatures. Consequently, the user has two options for putting the invention into practice.
The first option is to obtain cold-rolled strip with a surface quality that is very regularly greater than that of the usual strip. To this end the line retains the same descaling tools as are used on conventional installations, in particular prior to cold rolling. Those tools usually include, in succession:
a scale breaker in which the hot-rolled strip is flexed and slightly stretched, which fractures the layer of scale,
one or more mechanical descaling tools, which can include, alone or in cascade, a shot peening installation, a brushing installation, and an installation for spraying a fluid under pressure, the function of the last two tools being essentially to complete the removal of some of the residual scale detached by shot peening and any shot remaining on the surface of the strip, and
a chemical and/or electrolytic pickling installation, which can include one or more similar or different baths with composition(s) chosen in accordance with the usual parameters, such as the grade of the steel (which determines in part the composition and the behaviour of the scale), the type of surface finish required for the strip and the required treatment time. It may be necessary to cool the strip before it enters the pickling bath(s) if the temperature of the strip is relatively high on leaving the mechanical descaling facility (which can be the case in particular if it has undergone hot rolling followed by intermediate annealing).
The second option for using the method according to the invention entails being content with a regular surface quality of the strip that is merely comparable with that obtained by the conventional processes, from the point of view of encrustation of scale, but obtaining this result using a simplified descaling installation, which is therefore less costly to construct and to operate. The following options are available, for example:
dispensing with the scale breaker or limiting the severity of its action, which would be favourable to reducing the risk of surface cracks appearing on more fragile strip,
reducing the intensity of shot peening, or eliminating it completely, to obtain a strip surface that is less rough and less work-hardened, and
reducing the number of mechanical descaling tools and/or the number of pickling baths.
For grades for which an optimum surface quality is not necessarily required (for example steel for manufacturing automobile exhaust systems) it is even feasible to eliminate pickling entirely. In this case it is no longer necessary to cool the strip when it leaves the mechanical descaling facility, even if it has undergone hot rolling and intermediate annealing beforehand. Cold rolling can then be started at a higher temperature than usual, and possibly with a greater reduction ratio than usual (by adding extra mill stands or by increasing the reduction ratio of the existing mill stands). This opens up new possibilities in the fabrication of the product.